Conventional methods of assaying or purifying biological materials in samples, such as proteins in biological samples, exemplarily include a method in which proteins in a biological sample are captured on the surface of particles to which the proteins can bind, the particles are washed so that impurities excluding the target proteins in the biological sample are removed and the particles capturing the proteins are collected, and the amount of the proteins bound to the particles is calculated. The conventional methods also include a method in which particles capturing the protein are mixed in a protein-dissociating solution in order to dissociate the protein from the particles, and the proteins are purified by dissociating the proteins in the protein-dissociating solution.
In the above assay methods, magnetic particles are used because such magnetic particles are easily isolated and collected by the magnetic force. For example, Patent Literature 1 discloses magnetic silica particles including core particles of iron oxide and silica coating on the surface of the core particles. However, the magnetic substance of such magnetic particles is a ferromagnetic substance. This ferromagnetism causes the magnetic substance itself to form a temporary magnetic field even after removal of the magnetic field applied upon collection, leading to self-association of the particles. This results in poor washability and/or adverse effects on the subsequent operations (e.g. immune response).
For the purpose of preventing the self-association of a magnetic substance due to the ferromagnetism, Patent Literature 2 discloses magnetic silica particles with a superparamagnetic substance. If such magnetic particles have a small particle size, the particles contain a small amount of the magnetic substance and require a long time to be collected by the magnetic force. If the magnetic particles have a large particle size, the particles have a small specific surface area and can capture only a small amount of protein.